Kinetic fragility of Se-based binary chalcogenide glasses

Abstract

Kinetic fragility was determined for a number of compositions from the Ge–Se, As–Se and Te–Se chalcogenide glassy systems. Values of fragility indices were evaluated from both viscosity and enthalpy relaxation data. Comparison of the viscosity and enthalpy fragilities shows marked decoupling in cases of low-Ge/As-content compositions and the whole Te–Se glassy system. A possible explanation regarding the increased viscosity fragilities involves the influence of dynamic structural heterogeneity (density fluctuations and clustering of structural elements) on shear viscosity in the glass transition region. In particular, the observed decoupling seems to occur in case of the materials exhibiting moderate-to-high activation energy of relaxation processes and large structural cooperativity. The origin of the fragility decoupling may lie either in different molecular movements being involved in the two processes (viscous flow versus structural relaxation) or in their different manifestation in terms of volume and enthalpy behavior.